This invention relates to positioning a sensor on a microwave device, especially an applicator for treatment of a body by means of microwave electromagnetic energy, and also relates to an applicator including a sensor positioned thereon.
In our prior published application No. WO95/04385, the contents of which are incorporated herein by reference, we have disclosed apparatus for the treatment of menorrhagia which involves applying microwave electromagnetic energy at a frequency which will be substantially completely absorbed by the endometrium, monitoring the operating temperature to ensure that the endometrium tissue is heated to about 60xc2x0 and maintaining the application of the microwave energy for a period of time sufficient to destroy the cells of the endometrium.
The temperature is therefore important and a temperature sensor in the form of a thermocouple is used to monitor the temperature on an ongoing basis during application.
The problem which arises is that a thermocouple is constructed of metal and the application of microwave energy tends to cause direct heating of the thermocouple which leads to errors in the temperature readings. This general problem is discussed in S. B. Field and J. W. Hand xe2x80x9cAn Introduction to the Practical Aspects of Clinical Hyperthermiaxe2x80x9d at pages 459-465. As a result of the problems encountered with metallic sensors, it has been the practice to take readings either when the power is off, which precludes real-time measurement, or measurement has been by non-metallic sensors, such as fibre-optic sensors, which are much more expensive.
Microwave electromagnetic energy can be propagated either by coaxial waveguide or by tubular waveguide typically of circular cross-section.
The invention consists in a method of positioning on a microwave waveguide a sensor including an elongate metallic element comprising: selecting a tubular waveguide; determining the general orientation of the magnetic field generated during microwave transmission; and positioning the elongate metallic element substantially parallel to the orientation of the magnetic field.
With this arrangement, current should not be induced in the metallic element by the magnetic field and there should therefore be little or no interference with the parameter being sensed. Typically, the sensor will be a thermocouple sensing temperature and the inherent danger is interference by current flowing in the metal sheath of the thermocouple.
The invention also consists in a microwave applicator comprising a tubular waveguide which, on transmission of microwaves, generates an electric field orientated substantially perpendicular to the waveguide wall and a magnetic field substantially perpendicular to the electric field, and a sensor including an elongate metallic element, said elongate metallic substantially no current is induced in the metallic element of the sensor which would otherwise cause distortion.